1. Field of the invention
The present invention relates to a honeycomb heater having integrally formed and/or sintered electrodes and employing a honeycomb structure, and a method of manufacturing such a honeycomb heater.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as catalysts or substrates for catalysts which remove, for example, nitrogen oxides, in the exhaust gas of internal combustion engines, such as automobiles.
In recent years, there has been a desire to develop materials exhibiting greater mechanical strength and thermal resistance in hostile environments. To these ends, honeycomb structures manufactured by forming metal powders and then sintering the formed body have been put into practical use.
The present inventors have already proposed in U.S. patent application Ser. Nos. 07/545,509 and 07/545,697, a honeycomb heater which is comprised of a honeycomb structure with electrodes formed thereon to supply an electric current to the honeycomb structure. This honeycomb structure is produced by forming ceramic or metal powders into a desired honeycomb configuration and then sintering the formed honeycomb body.
In the above-described honeycomb heater, the electrodes are mounted on the outer peripheral wall or inside of the honeycomb structure by the conventional welding or brazing.
However, in the case where the electrodes are mounted on the outer peripheral wall of the honeycomb structure by welding, the portion of the thin outer peripheral wall of the honeycomb structure on which the electrodes are mounted is subjected to heat generated by the welding and may thus be damaged. Brazing requires a large number of manhours and is expensive. Furthermore, in the above-described honeycomb heater, since the electrodes are joined to the sintered honeycomb structure, the joined portions are exposed to heat in the exhaust gas during the use, deteriorating the durability of the honeycomb heater.